These problems are similar to the normalization problems that occur when designing a database. If you have tables such as Orders, OrderItems, and Items, you ne

These problems are similar to the normalization problems that occur when designing a database. If you have tables such as Orders, OrderItems, and Items, you need to account for the attributes of Items changing over time and how that will affect past Orders. The problems are potentially bigger in REST, but they're not necessarily new problems. The Cost of Goods Sold problem, in particular, has already been solved in a couple of different ways by the accounting profession. See http://en.wikipedia.org/wiki/Cost_of_goods_sold#Identification_conventions http://en.wikipedia.org/wiki/Cost_of_goods_sold#Identificati... for some details.